The present invention relates generally to muffler heat shields for small internal combustion engines, and more particularly, to a muffler heat shield that is attachable directly to the muffler.
Muffler guards or heat shields are utilized to prevent an operator or nearby bystanders of a small internal combustion engine from burning themselves on the hot muffler surfaces. The majority of heat shields in current use are of metal construction, either from stamped and formed sheet metal or from spaced frames made of welded steel wire. These designs are generally of an open construction to minimize surface area and trapped heat. Metal heat shields generally require a large air gap between the muffler and shield to reduce heat transfer rates and prevent high heat shield surface temperatures. Typically the shield is placed a distance from the muffler of approximately one inch. A problem with metal heat shields is that they occupy space that could be better used for additional muffler volume. In addition, metal heat shields tend to be relatively expensive per item.
Also known are several heat shield designs of plastic construction. These are also of an open construction and are spaced even further away from the muffler since they are formed generally of a low temperature thermoplastic material that cannot withstand a great amount of heat and still maintain structural integrity. The advantages of plastic heat shields over metal heat shields are twofold. First, plastic heat shields are easier to manufacture which generally results in a lower cost part. Second, plastic shields have a low heat transfer coefficient which results in less heat being transferred in and through the heat shield, which thereby results in a lower surface temperature. In addition, the transfer of heat from the heat shield to the skin occurs more slowly, thereby causing less damage to the skin in the event of skin contact with the heat shield surface.
Several methods are currently available for attachment of the heat shields to a muffler. The most common in use is to attach the heat shield to surrounding elements adjacent the muffler, such as the engine block, blower housing, or crankcase. These parts are generally much cooler than the muffler, but are not accurately located relative to the muffler, causing assembly and fit problems. Often, several parts are involved with several attachment points requiring that tolerance stack ups be taken into account. In addition, subassembly is not practical. This method is used for both metal and plastic shield construction.
A second method, applicable only to metal heat shields, is to spot weld or otherwise attach the heat shield permanently to the muffler itself. While this method could be utilized for subassembly, it has a disadvantage in that the assembly must be serviced as a complete unit, so that a damaged shield or muffler would require replacement of both. In addition, the direct metal attachment provides an undesirable heat path from the muffler to the heat shield. Heat shields attached in this manner must be spaced a relatively large distance from the muffler body, creating a large envelope for the muffler and heat shield assembly. In equipment in which space is at a premium, a larger heat shield envelope means less volume for the muffler for sound attenuation.
A third method, again applicable only to metal heat shields, is the use of metal spring clips to attach the heat shield to the muffler. This method is utilized for the muffler constructions having generally a crimped assembly of a number of stamped pieces, which provide a bead for attachment to the spring clip. The crimped bead appears as a parting line for the muffler, extending completely around the perimeter of the muffler. The clips attach to the inside perimeter of the heat shield and snap in place over the crimp bead. Although this method lends itself to subassembly, the clips are metal and therefore transfer heat to the shield. In addition, the crimp beads are not easily held to accurate limits, and tolerance stack ups make controlling part fits difficult.
Another known method of metal heat shield attachment is the use of intermediate supports, such as weld nuts or brackets that are permanently attached to the muffler. The heat shield then attaches to the supports, making the heat shield detachable and serviceable. The supports may also be removably attached. A disadvantage of this method is that it requires extra parts and extra welding operations, thereby adding to overall cost.
It is desirable to provide a heat shield design and a method of assembly that is low in cost, compact in size, highly functional from a safety standpoint, and easy to install or subassemble prior to installation to a muffler or to the engine.